Grain-oriented electrical steel sheet, and manufacturing method therefor

ABSTRACT

A method for manufacturing a grain-oriented electrical steel sheet, according to one embodiment of the present invention, comprises the steps of: manufacturing a hot rolled sheet by hot rolling a slab comprising, by wt %, 2.5-4.0% of Si, 0.03-0.09% of C, 0.015-0.040% of Al, 0.04-0.15% of Mn, 0.01% or less of S (excluding 0%) and 0.002-0.012% of N, and the balance of Fe and other inevitable impurities; cold rolling the hot rolled sheet to manufacture a cold rolled sheet; performing primary recrystallization annealing on the cold rolled sheet; and performing secondary recrystallization annealing on the cold rolled sheet for which the primary recrystallization annealing has been completed.

TECHNICAL FIELD

The present invention relates to a grain oriented electrical steel sheetand a method for manufacturing the same. More particularly, the presentinvention relates to a method for manufacturing a grain orientedelectrical steel sheet in which the formation of subgrain boundaries issuppressed and magnetism is improved by controlling a tension applied tothe steel sheet during the formation of an insulating coating layer.

BACKGROUND ART

In general, a grain oriented electrical steel sheet is a steel sheetcontaining an Si component, and refers to an electrical steel sheet thathas a texture in which crystal grain orientation is aligned in a{110}<001> direction and has extremely excellent magnetic properties ina rolling direction. Obtaining such a {110}<001> texture is possible bya combination of various manufacturing processes, and in particular, inaddition to components of a steel slab, a series of processes ofheating, hot rolling, hot rolled sheet annealing, primaryrecrystallization annealing, and secondary recrystallization annealingthe steel slab should be controlled very strictly. Specifically, sincethe grain oriented electrical steel sheet is to exhibit excellentmagnetic properties by a secondary recrystallized structure obtained byinhibiting the growth of primary recrystallized grains and selectivelygrowing crystal grains with {110} <001> orientation among crystal grainswhose growth is inhibited, growth inhibitors of primary recrystallizedgrains become more important. In the final annealing process, one of themain aspects of the grain oriented electrical steel sheet manufacturingtechnology among the crystal grains whose growth is inhibited is toallow crystal grains stably having a texture of {110}<001> orientationto grow preferentially. Examples of primary crystal grain growthinhibitors that may satisfy the above conditions and are currentlywidely used industrially include MnS, AlN, MnSe, etc. Specifically, theMnS, AlN, MnSe, etc., contained in the steel slab are reheated at hightemperature for a long time and dissolved, followed by hot rolling, andin the subsequent cooling process, components having an appropriate sizeand distribution are made into precipitates, which may be used as thegrowth inhibitor. However, this has a problem in that the steel slab isnecessarily heated to a high temperature. In this regard, efforts haverecently been made to improve the magnetic properties of the grainoriented electrical steel sheets by a method for heating a steel slab atlow temperature. To this end, a method for adding an antimony (Sb)element to a grain oriented electrical steel sheet has been proposed,but has been pointed out as a problem that noise quality of atransformer is poor due to the non-uniform and coarse crystal grain sizeafter the final high temperature annealing.

On the other hand, in order to minimize power loss of the grain orientedelectrical steel sheet, it is common to form an insulating film (ortensile coating layer) on a surface of the grain oriented electricalsteel sheet. In this case, the insulating film should basically havehigh electrical insulation and excellent adhesion to materials, and havea uniform color without defects in appearance. In addition, due to therecent strengthening of international standards for the noise of thetransformer and intensifying competition in related industries, researchon magnetic deformation (magnetostriction) is required to reduce noisein the insulating film of the grain oriented electrical steel sheet.Specifically, when a magnetic field is applied to the electrical steelsheet used as an iron core of the transformer, shrinkage and expansionare repeated to cause a trembling phenomenon, and vibration and noiseoccur in the transformer due to the trembling. In the case of thecommonly known grain oriented electrical steel sheet, by forming theinsulating film on a steel sheet and a Forsterite-based base film andapplying a tensile stress to the steel sheet using a difference in acoefficient of thermal expansion of the insulating film, an effect ofimproving core loss and reducing noise caused by magnetostriction ispromoted. However, there is a limit to satisfying a noise level ofhigh-grade grain oriented electrical steel sheet, which is recentlyrequired. Meanwhile, a wet coating method is known as a method forreducing the 90° magnetic domain of a grain oriented electrical steelsheet. Here, the 90° magnetic domain refers to an area havingmagnetization perpendicular to a direction in which a magnetic field isapplied, and the smaller the amount of the 90° magnetic domain, thesmaller the magnetostriction. However, in the general wet coatingmethods, there are disadvantages in that the effect of reducing noise byapplying tensile stress is insufficient, and that a thick film ofcoating thickness is required to be coated, resulting in deteriorationof space factor and efficiency of the transformer.

In addition, a coating method through vacuum deposition such as physicalvapor deposition (PVD) and chemical vapor deposition (CVD) has beenknown as a method for imparting high tensile properties to a surface ofa grain oriented electrical steel sheet. However, this coating method isdifficult to commercially produce, and the grain oriented electricalsteel sheet manufactured by this method has a problem in that theinsulating properties are poor.

DISCLOSURE Technical Problem

The present invention attempts to provide a method for manufacturing agrain oriented electrical steel sheet. More particularly, the presentinvention attempts to provide to a method for manufacturing a grainoriented electrical steel sheet in which the formation of subgrainboundaries is suppressed and magnetism is improved by controlling atension applied to the steel sheet during the formation of an insulatingcoating layer.

Technical Solution

An embodiment of the present invention provides a grain orientedelectrical steel sheet including: an electrical steel sheet basematerial containing, by wt %, 2.0 to 7.0% of Si, and 0.01 to 0.07 wt %of Sb, and the balance of Fe and other inevitable impurities; and aninsulating coating layer positioned on the electrical steel sheet basematerial, in which the insulating coating layer includes pores having aparticle size of 10 nm or more, the electrical steel sheet base materialhas a subgrain boundary that exists in an area A within 1500 μm in an RDdirection from a center of the pores and an area B within 50 to 100 μmfrom a surface of the electrical steel sheet base material toward aninside of the electrical steel sheet base material, the subgrainboundary has crystal orientation of an angle of 1° to 15° from {110}<001>, and an area fraction of the subgrain boundary in an ND crosssection is 5% or less.

In the subgrain boundary, a ratio y/z of a crystal grain length y in aTD direction to a crystal grain length z in the ND direction may be 1.5or less.

Goss crystal grains having the crystal orientation less than 1° from{110} <001> may be included in an area B of 50 to 100 μm from thesurface of the electrical steel sheet base material toward the inside ofthe electrical steel sheet base material, and a ratio LS/LG of anaverage particle size LS of the subgrain boundary to the averageparticle size LG of the Goss crystal grain in the ND cross section maybe 0.20 or less.

The number of pores having a particle size of 10 nm or more may be 1 to300 per 1 mm in the RD direction.

A fine grain interfacial layer may exist from the surface of theelectrical steel sheet toward the inside of the electrical steel sheetbase material, and the fine grain interfacial layer may have an averagegrain diameter of 0.1 to 5 μm.

The fine grain interfacial layer may have a residual stress of −10 to−1000 MPa in the RD direction.

A thickness of the fine grain interfacial layer may be 0.1 to 5 μm.

The grain oriented electrical steel sheet may further include a basecoating layer between the electrical steel sheet base material and theinsulating coating layer.

A residual stress of the base coating layer in the RD direction may be−50 to −1500 MPa.

A thickness of the base coating layer may be 0.1 to 15 μm.

A residual stress of the insulating coating layer in the RD direction is−10 to −1000 MPa.

A thickness of the insulating coating layer is 0.1 to 15 μm.

The electrical steel sheet base material may have a residual stress of 1to 50 MPa in the RD direction.

Another embodiment of the present invention provides a method formanufacturing a grain oriented electrical steel sheet including:manufacturing the grain oriented electrical steel sheet; applying aninsulating coating layer forming composition on the grain orientedelectrical steel sheet; and heat-treating the grain oriented electricalsteel sheet to form an insulating coating layer on the grain orientedelectrical steel sheet, in which, in the forming of the insulatingcoating layer, a tension applied to the steel sheet is 0.2 to 0.7kgf/mm².

For an entire length of the steel sheet, a maximum value MA and aminimum value MI of the tension may satisfy Equation 2 below.

[MI]≥0.5×[MA]  [Expression 2]

In the forming of the insulating coating layer, the heat treatment maybe performed at a temperature of 550 to 1100° C.

Advantageous Effects

According to a grain oriented electrical steel sheet according to anembodiment of the present invention, it is possible to improve magnetismby inhibiting subgrain boundaries that adversely affect the magnetism.

According to a grain oriented electrical steel sheet according to anembodiment of the present invention, it is possible to improve magnetismby increasing a residual stress of a base coating layer, an insulatingcoating layer, and a fine grain interfacial layer.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a TD cross section of a steel sheetaccording to an embodiment of the present invention.

FIG. 2 is an electron backscattered diffraction (EBSD) photograph of asteel sheet manufactured in Example 1.

FIG. 3 is a view showing a film tension calculation method using aradius of curvature.

FIG. 4 is a diagram illustrating a gradient in measurement of residualstress.

MODE FOR INVENTION

The terms first, second, third, and the like are used to describe, butare not limited to, various parts, components, areas, layers and/orsections. These terms are used only to distinguish a part, component,region, layer, or section from other parts, components, regions, layers,or sections. Accordingly, a first part, a component, an area, a layer,or a section described below may be referred to as a second part, acomponent, a region, a layer, or a section without departing from thescope of the present invention.

Terminologies used herein are to mention only a specific embodiment, anddo not to limit the present invention. Singular forms used hereininclude plural forms as long as phrases do not clearly indicate anopposite meaning. The meaning “including” used in the presentspecification concretely indicates specific properties, areas, integernumbers, steps, operations, elements, and/or components, and is not toexclude presence or addition of other specific properties, areas,integer numbers, steps, operations, elements, and/or components thereof.

When a part is referred to as being “above” or “on” other parts, it maybe directly above or on other parts, or other parts may be included inbetween. In contrast, when a part is referred to as being “directlyabove” another part, no other part is involved in between.

All terms including technical terms and scientific terms used hereinhave the same meaning as the meaning generally understood by thoseskilled in the art to which the present invention pertains unlessdefined otherwise. Terms defined in commonly used dictionaries areadditionally interpreted as having meanings consistent with relatedtechnical literature and currently disclosed content, and are notinterpreted in ideal or very formal meanings unless defined.

In addition, unless otherwise specified, % means wt %, and 1 ppm is wt%.

In an embodiment, further including additional elements means that thebalance of iron (Fe) is replaced and included as much as the additionalamount of the additional elements.

Hereinafter, an embodiment will be described in detail so that a personof ordinary skill in the art to which the present invention pertains caneasily implement the present invention. As those skilled in the artwould realize, the described embodiments may be modified in variousdifferent ways, all without departing from the spirit or scope of thepresent invention.

FIG. 1 is a schematic diagram of a TD cross section of a grain orientedelectrical steel sheet according to an embodiment of the presentinvention.

As illustrated in FIG. 1 , a grain oriented electrical steel sheet 100according to an embodiment of the present invention includes anelectrical steel sheet base material 10 and an insulating coating layer30 positioned on the electrical steel sheet base material 10.

The insulating coating layer 30 is formed by applying asolvent-containing insulating coating layer forming composition on asteel sheet and then heat-treating the steel sheet. In this case, as thesolvent volatilizes at a high temperature, some pores 31 are inevitablyformed in the insulating coating layer 30.

When the pores 31 are larger than 10 nm, the stress applied to the steelsheet is concentrated under the pore 31 to form a subgrain boundary 11.This has an adverse effect on magnetism compared to Goss crystal grain,which is a main crystal grain of the grain oriented electrical steelsheet, and therefore, is preferably suppressed as much as possible.

In an embodiment of the present invention, the formation of the subgrainboundary 11 needs to be suppressed as much as possible by analyzing apositional correlation between the pore 31 and the subgrain boundary 11and the cause of the formation of the subgrain boundary 11.

In FIG. 1 , the pore 31 and the subgrain boundary 11 are schematicallyrepresented.

As illustrated in FIG. 1 , the subgrain boundary 11 exists under thepores 31. All the subgrain boundaries 11 in the steel sheet basematerial 10 exist in a specific area under the pores 31. However, notall the subgrain boundaries 11 exist under all the pores 31, and theremay be the pores 31 having the subgrain boundaries 11 not existingthereunder.

Hereinafter, each configuration according to an embodiment of thepresent invention will be described in detail.

The electrical steel sheet base material 10 refers to a portion of thegrain oriented electrical steel sheet 100 excluding the base coatinglayer 20 and the insulating coating layer 30.

In an embodiment of the present invention, it is expressed by the pores31 in the insulating coating layer 30 and the subgrain boundaries 11 inthe electrical steel sheet base material 10, regardless of alloycomponents of the electrical steel sheet base material 10.Supplementally, the alloy components of the electrical steel sheet basematerial 10 will be described.

The electrical steel sheet base material 10 may contain, by wt %, 2.0 to7.0% of Si, 0.01 to 0.10% of Sn, 0.01 to 0.07% of Sb, 0.020 to 0.040% ofAl, 0.01 to 0.20% of Mn, 0.005% or less of C, 0.005% or less of N, and0.005% or less of S, and may include the balance of Fe and other inevitable impurities.

Si: 2.0 to 7.0 wt %

Silicon (Si) serves to reduce core loss by increasing a specificresistance of steel. When the Si content is too little, the specificresistance of the steel decreases, resulting in poor core lossproperties, and secondary recrystallization may be unstable due to thepresence of a phase transformation section during secondaryrecrystallization annealing. When the Si content is too much,brittleness may increase, and thus, cold rolling may be difficult.Therefore, the Si content may be adjusted within the above range. Morespecifically, Si may be contained in an amount of 2.5 to 5.0 wt %.

Sn: 0.01 to 0.10 wt %

Since tin (Sn) is a grain boundary segregation element that hindersgrain boundary movement, Sn promotes the generation of Goss crystalgrains having {110}<001> orientation as a crystal grain growth inhibitorto well develop secondary recrystallization. Therefore, Sn is animportant element for reinforcing crystal grain growth inhibitory force.

When the Sn content is too little, the effect is reduced, and when theSn content is too much, the grain boundary segregation occurs severely,resulting in increased brittleness of the steel sheet and sheet breakageduring rolling. Therefore, the Sn content may be adjusted within theabove range. More specifically, Sn may be contained in an amount of 0.02to 0.08 wt %.

Sb: 0.01 to 0.05 wt %

Antimony (Sb) is an element that promotes the formation of Goss crystalgrains in the {110}<001> orientation. When the Sb content is too little,a sufficient effect may not be expected as the Goss crystal grainformation promoter, and when the Sb content is too much, Sb issegregated on the surface, so the formation of the oxide layer may besuppressed and the surface defects may occur. Therefore, the Sb contentmay be controlled within the above range. More specifically, Sb may becontained in an amount of 0.02 to 0.04 wt %.

Al: 0.020 to 0.040 wt %

Aluminum (Al) is an element that acts as an inhibitor by finallybecoming a nitride in the form of AlN, (Al,Si)N, or (Al,Si,Mn)N. Whenthe Al content is too little, a sufficient effect as an inhibitor maynot be expected. On the other hand, when the Al content is too much, theeffect as an inhibitor is insufficient because Al-based nitridesprecipitate and grow too coarsely. Therefore, the Al content may beadjusted within the above range. More specifically, Al may be containedin an amount of 0.020 to 0.030 wt %.

Mn: 0.01 to 0.20 wt %

Manganese (Mn) has the effect of reducing core loss by increasingspecific resistance in the same way as Si, and is an element that reactswith nitrogen introduced by nitriding treatment together with Si to formprecipitates of (Al,Si,Mn)N, thereby suppressing the growth of primaryrecrystallized grains and causing the secondary recrystallization.However, when the Mn content is too much, Mn promotes austenite phasetransformation during hot rolling, thereby reducing the size of primaryrecrystallized grains and making secondary recrystallization unstable.In addition, when the Mn content is too little, Mn is an austeniteforming element, and increases an austenite fraction during hot-rollingreheating to increase the dissolved content of precipitates, so theeffect of preventing primary recrystallized grains from being tooexcessive through precipitate refinement and MnS formation duringre-precipitation may occur insufficiently. Therefore, the Mn content maybe adjusted within the above range.

C: 0.005 wt % or less

Carbon (C) is a component that does not greatly help improve themagnetic properties of the grain oriented electrical steel sheet in theembodiment according to the present invention, so C is preferablyremoved as much as possible. However, when C is included at a certainlevel or more, C has an effect of helping to form a uniformmicrostructure by promoting the austenite transformation of the steelduring the rolling process to refine the hot rolled structure during thehot rolling. The C content in the slab is preferably contained in anamount of 0.04 wt % or more. However, when the C content is excessive,since coarse carbides are formed and C is difficult to remove duringdecarburization, C may be contained in an amount of 0.07 wt %. Thedecarburization is performed in the primary recrystallization annealingprocess, and C is contained in an amount of 0.005 wt % or less in thegrain oriented electrical steel sheet base material finally manufacturedafter the decarburization.

N: 0.005 wt % or less

Nitrogen (N) is an element that refines crystal grains by reacting withAl or the like. When these elements are appropriately distributed, asdescribed above, they may be helpful to secure an appropriate primaryrecrystallized grain size by properly refining the structure after thecold rolling. However, when the content is excessive, the primaryrecrystallized grains are excessively miniaturized, and as a result, thedriving force that causes crystal grain growth during secondaryrecrystallization increases due to the fine crystal grains, so crystalgrains may grow in undesirable orientation. In addition, when the Ncontent is excessive, it is not preferable because it takes a lot oftime to remove it in the final annealing process. Therefore, the upperlimit of the nitrogen content may be set at 0.005 wt %. A nitrogencontent may increase due to nitration during the primaryrecrystallization process. In this case, since nitrogen is removed againduring the secondary recrystallization annealing process, the nitrogencontent in the slab and the final grain oriented electrical steel sheetbase material 10 may be the same.

S: 0.005 wt % or less

When the sulfur (S) content exceeds 0.005 wt %, S is re-dissolved andfinely precipitated when the hot rolled slab is heated, so a size ofprimary recrystallized grains is reduced and secondary recrystallizationinitiation temperature is lowered to deteriorate magnetism. In addition,since it takes a lot of time to remove S in a dissolved state in asecondary soaking section of the final annealing process, theproductivity of the grain oriented electrical steel sheet is reduced.Meanwhile, when the S content is as low as 0.005% or less, there is aneffect that the initial crystal grain size before the cold rolling iscoarsened, so the number of crystal grains having {110}<001> orientationnucleated in the deformation band in the primary recrystallizationprocess increases. Therefore, it is preferable that the S content is0.005 wt % or less in order to improve the magnetism of the finalproduct by reducing the size of the secondary recrystal grains.

The balance includes Fe and inevitable impurities. The inevitableimpurities are elements that are inevitably added in the manufacturingprocess of steelmaking and grain oriented electrical steel sheet, andsince the inevitable impurities are widely known, descriptions thereofwill be omitted. In an embodiment of the present invention, the additionof elements other than the above-described alloy components is notexcluded, and these elements may be variously contained within a rangethat does not impair the technical spirit of the present invention. Whenadditional elements are further contained, they are contained in placeof Fe which is the balance.

As illustrated in FIG. 1 , the subgrain boundary 11 exists in theelectrical steel sheet base material 10.

The subgrain boundary 11 is distinguished from other Goss crystal grainsexcept for the subgrain boundary in that the crystal orientation formsan angle of 1° to 15° from {110} <001>. Specifically, the Goss crystalgrains have the crystal orientation less than 1° from {110} <001>. Thecrystal orientation is represented by the Miller index.

In an embodiment of the present invention, the subgrain boundary 11 ispositioned under the pores 31. Specifically, the subgrain boundaryexists in the area A within 1500 μm in an RD direction from a center ofthe pores and an area B within 50 to 100 μm from the surface of theelectrical steel sheet base material toward the inside of the electricalsteel sheet base material. In FIG. 1 , the positions defined by theareas A and B are indicated by dotted rectangles. Specifically, all theareas of the subgrain boundary 11 may be included in positions definedas the areas A and B. In an embodiment of the present invention, thesubgrain boundary 11 exists only in the above-described area, and thesubgrain boundary 11 does not exist in the other part.

In an embodiment of the present invention, it is possible to improvemagnetism by inhibiting the subgrain boundary 11. Specifically, the areafraction of the subgrain boundary in the ND cross section may be 5% orless. When the area fraction of the subgrain boundary 11 is too large,this causes the deterioration in magnetism. More specifically, the areafraction of the subgrain boundary in the ND cross section may be 0.1 to5%. More specifically, it may be 1 to 3%. The ND cross section means aplane perpendicular to the ND direction.

The particle size of the subgrain boundary 11 is 1 to 500 nm, and it iscan be distinguished from the rest of the Goss crystal grains even withthe particle size. Specifically, the average particle size of the Gosscrystal grains excluding the subgrain boundaries may be 5 to 100 mm. Inthis case, it is the particle size in the crystal grain ND crosssection. More specifically, the particle size of the subgrain boundary11 may be 10 to 250 nm, and the average particle size of the Gosscrystal grains excluding the subgrain boundary may be 10 to 50 mm.

The ratio L_(S)/L_(G) of the average particle diameter L_(S) of thesubgrain boundary to the average particle diameter L_(G) of the Gosscrystal grain in the ND plane may be 0.20 or less. More specifically, itmay be 0.10 or less.

In an embodiment of the present invention, the particle size means adiameter of an imaginary circle having the same area as thecorresponding area.

In an embodiment of the present invention, the electrical steel sheetbase material 10 may have a residual stress of 1 to 50 MPa in the RDdirection. The reason why the residual stress exists in this range isdue to the base coating layer and the insulating coating layer 30existing above the electrical steel sheet base material 10. Due to thepresence of the residual stress in the above-described range, the filmtension is imparted to the base iron and the magnetism is improved.Specifically, the electrical steel sheet base material 10 may have aresidual stress of 16.0 to 30.0 MPa in the RD direction. The residualstress of the electrical steel sheet base material 10 may be obtained asa value that makes the sum of the residual stresses of the fine graininterfacial layer 12, the base coating layer 20, and the insulatingcoating layer 30 to be described later zero.

Σt_(i)σ_(i)=0

-   -   t_(i): Thickness of each layer    -   σ_(i): Residual stress of each layer    -   i: Base coating layer/fine grain interface layer/base steel        sheet

As illustrated in FIG. 1 , the fine grain interfacial layer 12 may existfrom the surface of the electrical steel sheet base material 10 towardthe inside of the electrical steel sheet base material. The fine graininterfacial layer 12 may have an average grain diameter of 0.1 to 5 μm.The fine grain interfacial layer 12 is formed due to the influence ofsurface energy non-uniformity.

A thickness of the fine grain interfacial layer 12 may be 0.1 to 5 μm.When the fine grain crystal layer 12 is too thick, the magnetismdeteriorates, so it is preferable to make the thickness of the finegrain crystal layer 12 thin. More specifically, the thickness of thefine grain interfacial layer 12 may be 0.5 to 3 μm.

The fine grain interfacial layer may have a residual stress of −10 to−1000 MPa in a RD direction. In this case, a negative sign means thestress that the fine grain interfacial layer 12 imparts to theelectrical steel sheet base material 10. More specifically, the finegrain interfacial layer 12 may have a residual stress of −10 to −500 MPain a RD direction. More specifically, the fine grain interfacial layer12 may have a residual stress of −400 to −500 MPa in a RD direction.

As illustrated in FIG. 1 , the grain oriented electrical steel sheet 100according to an embodiment of the present invention may further includea base coating layer 20 positioned between the electrical steel sheetbase material 10 and the insulating coating layer 30.

The base coating layer 20 forms a coating layer by reacting the oxidelayer formed in the primary recrystallization process with components inthe annealing separator. The base coating layer 20 improves adhesionbetween the insulating coating layer 30 and the electrical steel sheetbase material 10, and also imparts insulation to the grain orientedelectrical steel sheet 100 together with the insulating coating layer30.

The component of the base coating layer 20 is not particularly limited,but when MgO is included in the annealing separator component,forsterite Mg₂SiO₄ may be included. The base coating layer 20 may beomitted if necessary. That is, the electrical steel sheet base material10 and the insulating coating layer 30 may be in direct contact witheach other.

A thickness of the base coating layer may be 0.1 to 15 μm. When thethickness of the base coating layer 20 is too thin, it may notsufficiently perform the insulating role and the role of improvingadhesion to the insulating coating layer 30 described above. When thebase coating layer 20 is too thick, the space factor may decrease, andthe adhesion to the insulating coating layer 30 may deteriorate. Morespecifically, the thickness of the base coating layer 20 may be 0.5 to 3μm.

A residual stress of the base coating layer in the RD direction may be−50 to −1500 MPa. More specifically, the residual stress may be −500 to−1000 MPa. More specifically, the residual stress may be −760 to −1000MPa.

As illustrated in FIG. 1 , the insulating coating layer 30 is positionedon the electrical steel sheet base material 10. When the base coatinglayer 20 is positioned on the electrical steel sheet base material 10,the insulating coating layer 30 is positioned on the base coating layer20. The insulating coating layer serves to improve core loss byimparting insulation to the grain oriented electrical steel sheet 100and imparting tension to the electrical steel sheet base material 10.

The insulating coating layer 30 may use a material capable of impartinginsulation to the surface of the electrical steel sheet 100.Specifically, it may include phosphate (H₃PO₄).

The insulating coating layer 30 is formed by applying asolvent-containing insulating coating layer forming composition on asteel sheet and then heat-treating the steel sheet. In this case, as thesolvent volatilizes at a high temperature, some pores 31 are inevitablyformed in the insulating coating layer The pore 31 means a state inwhich nothing exists in the corresponding part, that is, an empty space.

The number of pores having a particle size of 10 nm or more may be 1 to300 per 1 mm in the RD direction. More specifically, 1 to 30 pores mayexist per 1 mm. In this case, the particle size of the pores may bemeasured based on the ND plane or the TD plane. The number of pores maybe measured based on the TD plane.

There are 1 to 30 subgrain boundaries per pore with a particle size of10 nm or more. As described above, the subgrain boundary 11 may notexist in the areas A and B under the pore 31, and it is also possiblethat two or more subgrain boundaries 11 exist. However, the subgrainboundary 11 may not exist other than the areas A and B under the pores31.

A thickness of the insulating coating layer is 0.1 to 15 μm. When thethickness of the insulating coating layer 30 is too thin, theabove-described insulating role may not be sufficiently performed. Whenthe base coating layer 30 is too thick, the space factor may decrease,and the adhesion with the electrical steel sheet base material 10 maydecrease. More specifically, the thickness of the insulating coatinglayer 30 may be 1.0 to 5.0 μm.

The residual stress of the insulating coating layer 30 in the RDdirection is −10 to −1000 MPa. More specifically, the residual stressmay be −70 to −500 MPa.

A method for manufacturing a grain oriented electrical steel sheetaccording to an embodiment of the present invention includesmanufacturing the grain oriented electrical steel sheet; applying aninsulating coating layer forming composition on the grain orientedelectrical steel sheet; and heat-treating the grain oriented electricalsteel sheet to form an insulating coating layer forming composition onthe grain oriented electrical steel sheet.

Hereinafter, each step will be described in detail.

First, the grain oriented electrical steel sheet is manufactured. Inthis case, the grain oriented electrical steel sheet may use the grainoriented electrical steel sheet in which the base coating layer 20 isformed or not, and only the electrical steel sheet base material 10exists.

The grain oriented electrical steel sheet on which the base coatinglayer is not formed may be manufactured in various ways. For example, amethod for adjusting components of an annealing separator, or formingthe base coating layer 20 and then removing the base coating layer 20 byphysical or chemical methods may be used.

In an embodiment of the present invention, there is a technical featurein adjusting the tension applied to the steel sheet in the step offorming the insulating coating layer, and various methods known in theart can be used for manufacturing the grain oriented electrical steelsheet.

Hereinafter, an example of a method for manufacturing a grain orientedelectrical steel sheet before forming an insulating coating layer willbe described.

The method for manufacturing a grain oriented electrical steel sheetaccording to an embodiment of the present invention may further include:manufacturing a hot rolled sheet by hot rolling a slab; manufacturing acold rolled sheet by cold-rolling the hot rolled sheet; performingprimary recrystallization annealing on the cold rolled sheet; andperforming secondary recrystallization annealing on the cold rolledsheet for which the primary recrystallization annealing has beencompleted.

The slab may contain 2.0 to 7.0 wt % of Si, 0.01 to 0.10 wt % of Sn,0.01 to 0.07 wt % of Sb, 0.020 to 0.040 wt % of Al, 0.01 to 0.20 wt % ofMn, 0.04 to 0.07 wt % of C, 10 to 50 wtppm of N, and 0.001 to 0.005 wt %of S, and the balance of Fe and other inevitable impurities.

First, the slab is hot-rolled to manufacture the hot rolled sheet.

Hereinafter, since the slab alloy components are the same as those ofthe electrical steel sheet base material 10 except for the C content,duplicate descriptions thereof will be omitted.

A step of heating the slab to 1230° C. or less may be further includedbefore the step of manufacturing the hot rolled sheet. Through thisstep, the precipitate may be partially dissolved. In addition, since thecoarse growth of the columnar structure of the slab is prevented, it ispossible to prevent cracks from occurring in the width direction of theplate in the subsequent hot rolling process, thereby improving the realyield. When the slab heating temperature is too high, the melting of thesurface of the slab may repair the heating furnace and shorten the lifeof the heating furnace. More specifically, the slab may be heated to1130 to 1200° C. It is also possible to hot-roll a continuously castslab as it is without heating the slab.

In the step of manufacturing the hot rolled sheet, the hot rolled sheethaving a thickness of 1.8 to 2.3 mm may be manufactured by hot rolling.

After manufacturing the hot rolled sheet, a step of hot rolled sheetannealing of the hot rolled sheet may be further included. The step ofannealing the hot rolled sheet may be performed by heating to atemperature of 950 to 1,100° C., cracking at a temperature of 850 to1,000° C. and then cooling.

Next, the cold rolled sheet is manufactured by cold-rolling the hotrolled sheet.

The cold rolling may be performed through one-time steel cold rolling orthrough a plurality of passes. It may give a pass aging effect throughwarm rolling at a temperature of 200 to 300° C. one or more times duringrolling, and may be manufactured to a final thickness of 0.14 to 0.25mm. The cold rolled sheet is subjected to decarburization andrecrystallization of deformed structure in the primary recrystallizationannealing process and nitriding treatment through nitriding gas.

Next, the cold rolled sheet is subjected to the primaryrecrystallization annealing.

The decarburization or nitriding may be performed in the primaryrecrystallization annealing process.

The primary recrystallization annealing step may be performed at atemperature of 800 to 900° C. When the temperature is too low, theprimary recrystallization may not be performed or the nitriding may notbe performed smoothly. When the temperature is too high, the primaryrecrystallization grows too large, causing the poor magnetism.

For the decarburization, it may be performed in an atmosphere having anoxidation capacity (PH₂O/PH₂) of 0.5 to 0.7. By the decarburization, thesteel sheet may contained in an amount of 0.005 wt % or less of carbon,more specifically, 0.003 wt %.

Next, the annealing separator is applied to the cold rolled sheet forwhich the primary recrystallization annealing has been completed,followed by secondary recrystallization annealing. Various separatorsmay be used as the annealing separator. For example, the annealingseparator containing MgO as a main component may be applied. In thiscase, after the secondary recrystallization annealing, the base coatinglayer 20 containing forsterite is formed.

The purpose of the secondary recrystallization annealing is to form{110}<001> texture by secondary recrystallization and to removeimpurities that harm magnetic properties. As a method of secondaryrecrystallization annealing, in the temperature rising section beforethe secondary recrystallization occurs, a mixed gas of nitrogen andhydrogen is maintained to protect nitride, which is a grain growthinhibitor, so the secondary recrystallization may develop well, andafter the completion of the secondary recrystallization, it may bemaintained for a long time in a 100% hydrogen atmosphere to removeimpurities.

After the secondary recrystallization annealing step, a flatteningannealing process may be included.

Returning to the description of the process of manufacturing the grainoriented electrical steel sheet according to an embodiment of thepresent invention, the insulating coating layer forming composition isapplied on the grain oriented electrical steel sheet. In an embodimentof the present invention, the insulating coating layer formingcomposition may be used in various ways, and is not particularlylimited. For example, the insulating coating layer forming compositioncontaining phosphate may be used.

Next, the heat treatment is performed on the grain oriented electricalsteel sheet to form the insulating coating layer on the grain orientedelectrical steel sheet.

In this case, as the solvent volatilizes at a high temperature duringthe heat treatment process, some pores 31 are inevitably formed in theinsulating coating layer 30. In this case, the stress applied to thesteel sheet is concentrated under the pores 31 to form the subgrainboundary 11. In an embodiment of the present invention, the formation ofthe subgrain boundary 11 is inhibited as much as possible by adjustingthe tension applied to the steel sheet during the formation of theinsulating coating layer.

Specifically, the tension applied to the steel sheet in the step offorming the insulating coating layer is 0.20 to 0.70 kgf/mm².

In this case, when the tension applied to the steel sheet is too small,scratches may occur on the surface, resulting in poor corrosionresistance. When the tension applied to the steel sheet is too large, alarge amount of subgrain boundaries 11 may be formed, which mayadversely affect magnetism. More specifically, it may be 0.20 to 0.50kgf/mm². More specifically, it may be to 0.47 kgf/mm². In this case, thetension is the average tension in the longitudinal direction of thesteel sheet measured at the exit side of the heat treatment process.

In the step of forming the insulating coating layer, the tension appliedalong the longitudinal direction (RD direction) of the steel sheet maybe different. In an embodiment of the present invention, the residualstress applied to each layer may be appropriately controlled byminimizing the difference between the maximum value MA and the minimumvalue MI of the tension over the entire length of the steel sheet, andthe formation of the subgrain boundary 11 may be inhibited.

Specifically, for an entire length of the steel sheet, the maximum valueMA and the minimum value MI of the tension may satisfy Equation 2 below.

[MI]≥0.5×[MA]  [Expression 2]

When Equation 2 is not satisfied and there is a large deviation intension along the length direction (RD direction) of the steel sheet,the non-uniformity increases locally, the residual stress is notappropriately controlled, and a large amount of subgrain boundaries 11are formed.

In the conventional case, there is a problem in that the deviation intension is large in the longitudinal direction (RD direction) of thesteel sheet due to the large change in line speed in the flatteningannealing process, resulting in locally increased non-uniformity. Indetail, laser welding is performed by minimizing the line speed to bonda preceding coil tail part and a following coil top part at the entranceof the flattening annealing. When welding is completed, there is a largedeviation in tension because the line speed increases to improve theproductivity of the final product. More specifically, since the changewidth in speed change width of a bridle roll and a hearth roll increasesaccording to the change in the line speed, the large deviation intension may occur in the length direction (RD direction) of the steelplate at high temperature, which is inevitably accompanied duringflattening annealing, and the residual stress may not be appropriatelycontrolled due to the local increase in non-uniformity, so the minimumvalue MI of the tension is inevitably less than 0.5×[MA].

There are many methods to reduce the difference between the maximumvalue MA and the minimum value MI of the tension, but in an embodimentof the present invention, for example, a method for controlling a bridleroll and controlling a speed of a hearth roll may be used. In detail,the bridle roll control is a method of controlling feedback tension byfollowing a value of a tension meter. More specifically, it is a methodof controlling a speed of a bridle roll to reduce the difference betweenthe maximum value and the minimum value of tension. Also, in detail, thehearth roll control is a method of controlling feedforward tensionfollowing a speed of a bridle roll. More specifically, in order toreduce the difference between the maximum value and the minimum value ofthe tension, it may be adjusted by controlling the tension to decreaseas the speed of the hearth roll increases. In an embodiment of thepresent invention, even if the line speed is varied in the flatteningannealing process, it is possible to reduce the difference between themaximum value MA and the minimum value MI while adjusting the tensionwithin a specific range.

In the step of forming the insulating coating layer, the heat treatmenttemperature may be 550 to 1100° C. At the above-described temperature,fewer pores 31 are generated, and residual stress of the insulatingcoating layer 30 may be appropriately applied.

The following illustrates the preferred Examples and ComparativeExamples of the present invention . However, the following Examples areonly embodiments of the present invention, and the present invention isnot limited to the following Examples.

After the steel that contains 3.4 wt % of Si, 0.05 wt % of Sn, 0.02 wt %of Sb, 0.02 wt % of Al, 0.10 wt % of Mn, 0.05 wt % of C, 0.002 wt % ofN, and 0.001 wt % of S, and contains the balance of Fe and otherinevitable impurities as the rest components are vacuum melted, an ingotwas made. Thereafter, the ingot was heated at 1150° C. for 210 minutes,followed by hot rolling to manufacture a hot rolled sheet having athickness of 2.0 mm. After pickling, it was cold-rolled to a thicknessof 0.220 mm.

The cold rolled sheet was maintained in a humid atmosphere of 50 v % ofhydrogen and 50 v % of nitrogen and an ammonia mixed gas atmosphere at atemperature of about 800 to 900° C., and was subjected todecarburization and nitriding annealing heat treatment so that thecarbon content was 30 ppm or less and the total nitrogen contentincreased to 130 ppm or more.

The steel sheet was applied with MgO as an annealing separator, andfinally annealed into a coil shape. The final annealing was performed ina mixed atmosphere of 25 v % of nitrogen and 75 v % pf hydrogen up to1200° C., and after reaching 1200° C., the steel sheet was kept in a100% hydrogen atmosphere for more than 10 hours and then cooled in afurnace.

The steel sheet was applied with an insulation coating layer formingcomposition containing phosphate and silica, and heat-treated at atemperature of about 820° C. for 2 hours to form an insulation coatinglayer.

When forming the insulating coating layer, an average tension at theexit side was adjusted as shown in Table 1 below.

The pores, the subgrain boundaries, and other crystal graincharacteristics of the manufactured grain oriented electrical steelsheet were summarized in Table 1, and the properties and core loss ofthe interfacial layer, the base coating layer, and the insulatingcoating layer were summarized in Table 2.

It was confirmed that the position of the subgrain boundary exists onlyin a specific area under the pore.

As for the number of pores, only pores with a particle size of 10 nm ormore were measured.

The subgrain boundary fraction was measured by an electron backscatterdiffraction (EBSD) method for volume per unit area.

The core loss W_(17/50) and magnetic flux density B₈ were measuredimmediately after the formation of the insulating coating layer andafter heat treatment at 820° C. for 2 hours assuming stress reliefannealing. The core loss was measured under the condition of 1.7 Tesla,50 Hz using the single sheet measurement method. In addition, themagnetic flux density induced in a magnetic field of 800 A/m wasmeasured.

The residual stress of the insulating coating layer was measured using a3D curvature measuring instrument (ATOS core 45). It was measured byremoving only the insulating coating layer on one side and measuring thebending amount of the steel sheet.

The insulation was measured above the coating using a Franklin measuringinstrument according to the ASTM A717 international standard.

The corrosion resistance indicates an area of rust generated on thesurface under the condition of 35° C., 5% NaCL, 8 hours according to JISZ2371 international standard. The diagram below is a film tensioncalculation method using the radius of curvature (reference M. Bielawskiet all., Surf. & Coat. Techno., 200 (2006) 2987). The film tension maybe calculated from the measured image using the 3D scanner software. Rvalues may be measured for specimens before (R2) and after (R1) removalof the phosphate coating layer.

$\sigma_{f} = {\frac{E_{s}}{6( {1 - v_{s}} )} \times \frac{t_{s}^{2}}{t_{f}} \times ( {\frac{1}{R_{2}} - \frac{1}{R_{1}}} )}$

-   -   1. σ_(f): Film tension    -   2. E_(s): Base layer Young's rate (electrical steel sheet:        176900 MPa)    -   3. U_(s): Base layer Poission ratio (electrical steel sheet:        0.3)    -   4. t_(f): Film thickness (mm)    -   5. t_(s): Base specimen thickness (mm)    -   6. R₂: Radius of curvature of base layer after film coating (mm)    -   7. R₁: Radius of curvature of base layer before film coating        (mm)

The residual stress of the base coating layer and the fine graininterfacial layer was measured using synchrotron XRD equipment. TheX-ray residual stress measurement method uses a distance between latticeplanes of crystal grains as a strain gauge. When a sample is in a stateof stress, a change occurs in a distance between the lattice planesdepending on the stress direction and a relative angle of the crystalplanes. It may be said that a distance between a lattice planes parallelto the tensile direction, that is, lattice planes with Ψ=0°, is smallerwhen the stress is zero due to the Poisson effect, and a distancebetween lattice planes with an inclined Ψ angle to the tensile directionis greater than when the stress is zero. The X-ray residual stressmeasures the peak shift according to a tilting angle Ψ. Therefore, theX-ray residual stress calculation follows the sin²Ψ method and may beexpressed as the following Expression.

-   -   Biaxial stress system: ϵ_(Ψ)−ϵ₃=(σ_(φ)/E) (1+v) sin²Ψ    -   X-ray measurement: ϵ_(Ψ)−ϵ₃=(d_(Ψ)−d₀)/d₀−(d_(z)−d₀)/d₀    -   ϵ_(Ψ)−ϵ₃=(d_(Ψ)−d_(z))/d₀−(d_(Ψ)−d_(z))/d_(z)    -   In summary, (d_(Ψ)−d_(z))/d_(z)=(σ_(φ)/E) (1+v) sin²Ψ    -   d_(Ψ): d-spacing of lattice plane arranged in Ψ direction as        lattice plane direction    -   d_(z): d-spacing of lattice plane arranged in direction in which        lattice plane direction is perpendicular to sample surface    -   d₀: d-spacing of stress-free lattice plane

TABLE 1 Tension at Whether or not Area fraction of exit side Expression2 is subgrain boundary (kgf/mm²) satisfied (%) Example 1 0.20 ◯ 0.01Example 2 0.34 ◯ 0.01 Example 3 0.42 ◯ 0.06 Example 4 0.44 ◯ 0.22Example 5 0.46 ◯ 0.03 Example 6 0.48 ◯ 0.17 Example 7 0.58 ◯ 0.50Example 8 0.60 ◯ 1.12 Example 9 0.70 ◯ 1.21 Comparative 0.55 X 8.82Example 1 Comparative 0.10 ◯ 9.10 Example 2 Comparative 0.77 ◯ 9.05Example 3 Comparative 0.86 ◯ 11.52 Example 4 Comparative 0.95 ◯ 22.30Example 5 Comparative 0.70 X 33.50 Example 6

TABLE 2 Average grain diameter 2.5 μm of fine grain Base coatingInsulating Steel sheet interfacial layer layer coating layer basematerial Residual Residual Residual Residual stress stress stress stressin RD in RD in RD in RD Thickness direction Thickness directionThickness direction Thickness direction (μm) (MPa) (μm) (MPa) (μm) (MPa)(μm) (MPa) Example 1 1.4 −480 1.1 −914 1.9 −325 220 20.6 Example 2 1.4−477 1.1 −895 1.9 −312 220 2.01 Example 3 1.4 −441 1.1 −868 1.9 −267 22018.6 Example 4 1.4 −414 1.1 −867 1.9 −272 220 18.4 Example 5 1.4 −4811.1 −858 1.9 −169 220 17.4 Example 6 1.4 −467 1.1 −870 1.9 −149 220 16.9Example 7 1.4 −454 1.1 −853 1.9 −94 220 15.7 Example 8 1.4 −427 1.1 −8331.9 −82 220 14 Example 9 1.4 −415 1.1 −780 1.9 −77 220 14.2 Comparative1.4 −247 1.1 −523 1.9 −37 220 8.8 Example 1 Comparative 1.4 −345 1.1−752 1.9 −55 220 9.7 Example 2 Comparative 1.4 −315 1.1 −524 1.9 −45 2209.9 Example 3 Comparative 1.4 −245 1.1 −447 1.9 −26 220 7.9 Example 4Comparative 1.4 −194 1.1 −398 1.9 −7 220 6.4 Example 5 Comparative 1.4−190 1.1 −225 1.9 −6 220 4.7 Example 6

TABLE 3 Core loss Magnetic flux (W17/50, density Insulation CorrosionW/kg) (B8, T) (mA) resistance Example 1 0.735 1.935 35 — Example 2 0.7391.935 55 — Example 3 0.752 1.934 30 — Example 4 0.753 1.935 35 — Example5 0.76 1.933 42 — Example 6 0.761 1.932 32 — Example 7 0.772 1.928 55 —Example 8 0.77 1.93 55 — Example 9 0.782 1.927 42 0.7 Comparative 0.8471.921 95 5.5 Example 1 Comparative 0.844 1.922 360 8.2 Example 2Comparative 0.843 1.923 277 7.7 Example 3 Comparative 0.912 1.915 345 9Example 4 Comparative 0.998 1.88 678 15 Example 5 Comparative 1.0521.876 850 42.3 Example 6

As shown in Tables 1 to 3, when the tension is properly controlled inthe process of forming the insulating coating layer, it can be seen thatthe subgrain boundary is suppressed, and the residual stress of the finegrain interfacial layer, the base coating layer, and the insulatingcoating layer increases, and the magnetism, insulation, and corrosionresistance are improved. On the other hand, when the tension is notproperly controlled during the formation of the insulating coatinglayer, it can be seen that a large amount of subgrain boundary isformed, and the magnetism, the insulation, or the corrosion resistanceis poor.

The present invention is not limited to the embodiments, but may bemanufactured in a variety of different forms, and those of ordinaryskill in the art to which the present invention pertains will understandthat the present invention may be implemented in other specific formswithout changing the technical spirit or essential features of thepresent invention. Therefore, it should be understood that theabove-mentioned embodiments are exemplary in all aspects but are notlimited thereto.

Descryption of Symbols 100: Grain oriented electrical steel sheet  10:Electrical steel sheet base material  11: Subgrain boundary 12: Finegrain interfacial layer  20: Base coating layer 30: Insulating coatinglayer  31: Pore

1. A grain oriented electrical steel sheet, comprising: an electricalsteel sheet base material containing, by wt %, 2.0 to 7.0% of Si and0.01 to 0.07 wt % of Sb, and the balance of Fe and other inevitableimpurities; and an insulating coating layer positioned on the electricalsteel sheet base material, wherein the insulating coating layer includespores having a particle size of 10 nm or more, the electrical steelsheet base material has a subgrain boundary that exists in an area Awithin 1500 μm in an RD direction from a center of the pores and an areaB within 50 to 100 μm from a surface of the electrical steel sheet basematerial toward an inside of the electrical steel sheet base material,the subgrain boundary has an angle of 1° to 15° from crystal orientationof {110}<001>, and an area fraction of the subgrain boundary in an NDcross section is 5% or less.
 2. The grain oriented electrical steelsheet of claim 1, wherein: in the subgrain boundary, a ratio y/z of acrystal grain length y in a TD direction to a crystal grain length z inthe ND direction is 1.5 or less.
 3. The grain oriented electrical steelsheet of claim 1, wherein: a Goss crystal grain less than 1° from thecrystal orientation of {110} <001> is included in an area B of 50 to 100μm from the surface of the electrical steel sheet base material towardthe inside of the electrical steel sheet base material, and a ratioLs/LG of an average particle size Ls of the subgrain boundary to theaverage particle size LG of the Goss crystal grain in the ND crosssection is 0.20 or less.
 4. The grain oriented electrical steel sheet ofclaim 1, wherein: a fine grain interfacial layer exists from the surfaceof the electrical steel sheet toward the inside of the electrical steelsheet base material, and the fine grain interfacial layer has an averagegrain diameter of 0.1 to 5 μm.
 5. The grain oriented electrical steelsheet of claim 4, wherein: the fine grain interfacial layer has aresidual stress of −10 to −1000 MPa in the RD direction.
 6. The grainoriented electrical steel sheet of claim 4, wherein: a thickness of thefine grain interfacial layer is 0.1 to 5 μm.
 7. The grain orientedelectrical steel sheet of claim 1, further comprising: a base coatinglayer between the electrical steel sheet base material and theinsulating coating layer.
 8. The grain oriented electrical steel sheetof claim 7, wherein: a residual stress of the base coating layer in theRD direction is −50 to −1500 MPa.
 9. The grain oriented electrical steelsheet of claim 7, wherein: a thickness of the base coating layer is 0.1to 15 μm.
 10. The grain oriented electrical steel sheet of claim 1,wherein: a residual stress of the insulating coating layer in the RDdirection is −10 to −1000 MPa.
 11. The grain oriented electrical steelsheet of claim 1, wherein: a thickness of the insulating coating layeris 0.1 to 15 μm.
 12. The grain oriented electrical steel sheet of claim1, wherein: the electrical steel sheet base material has a residualstress of 1 to 50 MPa in the RD direction.